Why Are Powders Complex?

Mechanisms of Particle Interaction

Further insight into the behaviour of the bulk powder can be gained by understanding the mechanisms of interaction that exist between particles.
There are a number of effects that contribute to the ease of movement of one particle relative to another. Understanding each effect, and how they can be engineered to work to our advantage provides great opportunity in both product development and process optimisation.

Mechanisms that restrict particle movement: -

Friction

Particles with a smoother surface will generally have a lower frictional interaction and flow more easily than those that are rougher, assuming all other features are identical.

Mechanical Interlocking

Particles of a certain shape can mechanically interlock and resist flow.

Inter-particulate forces of cohesion

Inter-particulate forces act between contacting particles, and those in close proximity.

Liquid Bridging

Liquid has the ability to bridge between particles, creating capillary bonds and reducing particle independence.

All mechanisms above act to restrict particle-particle independence, and generally speaking, the stronger their influence, can result in poorer powder flowability. However, powders with high levels of cohesion, irregular morphology and high surface friction still can be observed to flow, so there is clearly a significant motivating force that causes particles to move.

Mechanisms that promote particle movement: -

Gravity

Gravitational forces are often the only motive force acting on the particle.

Ignoring for a moment that particles in motion will have inertia, the primary motivating force acting on a loosely packed stationary particle is that due to gravity – its weight. Therefore, the ability of a particle to begin to flow is largely dependent on the strength of the gravitational force acting on it. It is for this reason that powders containing particles of large size, or consisting of material that has a high density, tend to flow better when loosely packed, as the particle’s individual mass, and therefore the gravitational force acting on it, is high.

The behaviour of the bulk powder is influenced by all mechanisms of particle interaction. However, the influence of each will depend on the properties of the powder and the environmental / process conditions imposed.